Although they reside in different cell types, all herpes viruses can enter a state of quiescence termed latency. When these viruses are latent, they are resistant to antiviral therapies that attack replicating viruses. As a result, existing drugs treat symptoms of active infections, but do not clear latent virus, and, therefore, do not cure herpes virus infections. Consequently, it is critically important to advance our understanding of the mechanisms underlying latency; indeed, this is one of the major outstanding questions in our field. Human cytomegalovirus (HCMV) resides in a latent state in cells of the myeloid lineage. Although the virus likely reactivates frequently from latency, it is readily controlled with no known consequences to healthy people. However, reactivation can cause severe morbidity and mortality when it re-emerges from latency in people with compromised or immature immune systems. Thus, it is critical to develop the ability to control and eventually eradicate latent virus, and this capability will come from a deeper understanding of latency. My long term objective is to employ a validated cell culture model of latency -- peripheral blood monocytes -- to investigate the molecular mechanisms underlying the initiation, maintenance and exit from latency. We will carry out our investigations primarily with clinical isolates of HCMV; and our technical approach will combine genetics, molecular biology and proteomics. My specific aims are designed to employ our cell culture model of latency to (i) investigate the effect of cell phenotype on HCMV latency; (ii) study the role of viral protein-coding genes with the potential to function during latency; and (iii) test the hypothesis that viral and/or cell-coded miRNAs function in HCMV latency. This is an opportune time to study HCMV latency, because we now appreciate many of the critical differences between clinical versus laboratory isolates of the virus that cause them to behave differently in experimental infections, experimental systems have been developed to investigate latency in cultured cells, and new biological components likely to play important roles in latency have been recently identified.